MOPA apparatus delivering 1064-nm picosecond pulses is now extensively used for operations such as laser marking. The range of operations that can be addressed by such MOPA apparatus is greater, the greater the energy and average power of the pulse. A preferred arrangement of such apparatus includes a mode-locked fiber-laser or all-fiber MOPA, providing seed-pulses for amplification. This typically uses ytterbium-doped (Yb-doped) gain-fiber. The seed-pulses are then amplified in a diode-pumped bulk solid-state amplifier, typically having a gain element of Nd:YVO4. Typically, the gain-element is end-pumped, i.e., with a pump-beam coaxial with a seed-pulse beam.
Typical prior-art design of diode-pumped Nd:YVO4 solid-state amplifier, end-pumped with pump-radiation power greater than or equal to about 50 Watts (W), revolves around distributing absorption of pump-radiation in order to distribute the heat-load in the crystal and minimize the effects of thermal lensing. In addition, the pump-spot size (minimum radius in the gain-element of a beam from the diode-laser) is normally chosen to maximize a spatial overlap between the pump-beam and the seed for a given brightness of diode-laser radiation in order to optimize extraction efficiency. These amplifiers are usually designed for power amplification and typically provide a gain-factor less than 10, usually between about 1.5 and 3.
Initially, Nd:YVO4 gain-elements were pumped with diode-laser radiation having a wavelength of about 808 nm. Nd:YVO4, however, is a uniaxial strongly birefringent crystal material, with radiation at the 808 nm peak being much more strongly absorbed in the crystal c-axis that in the crystal a-axis. This caused problems with crystal breakage under high power due to differential absorption. This was mitigated by pumping at an “off-peak” wavelength, for example 815 nm at which absorption is the same in both crystal-axes albeit less than at the 808-nm peak.
In later developments, pumping has been effected at another Nd:YVO4 peak-absorption wavelength of 880 nm. This longer wavelength takes advantage of a reduced quantum defect (difference between pump-photon energy and emitted-photon energy) to reduce heat-load and allow for an increase in pump-power. At 880 nm, however, there is a polarization (crystal-axis) dependence of absorption similar to that at 808 nm. This has led to a selection of 878.6 nm as a compromise pump-radiation wavelength at which the crystal-axis absorption-difference is less than at the 888 nm peak, albeit not zero. A brief description of 878.6-nm pumping of Nd:YVO4 is presented in very general terms in an article “VBG Upper-State Pumping Benefits DPSS Lasers”, in Laser Focus World, Volume 49, Issue No. 3.
This 878.6 nm (or 888-nm) pumping-wavelength is typically combined with an increase in the pump-spot radius and brightness to optimize the absorption length of radiation in the crystal. This reduces the maximum temperature reached in the crystal and associated thermal aberrations. Further, overlap between the pump-beam and the seed-pulse beam is optimized along the entire length of the gain-element to maximize extraction-efficiency (emission output-power/input pump-power).
This is illustrated in FIG. 1, which is a reproduction of a Gaussian ray-trace schematically illustrating the form and dimensions of the focused pump-radiation beam relative to the form and dimensions of a seed-pulse beam in a prior-art amplifier Nd:YVO4 gain-element. Here, it should be noted that longitudinal dimensions are shown foreshortened relative to lateral dimensions for convenience of illustration. A focused pump beam is depicted bounded by bold solid lines having the well-known hyperbolic form of a focused, Gaussian-propagation beam. A collimated seed-pulse beam is depicted by bold dashed lines.
The beam-waist diameter is 2ω0, i.e., twice the minimum beam-radius ω0. Typically the beam-waist diameter would be between about 0.8 millimeters (mm) and about 1.5 mm. A beam-waist length LW is measured between points at which the focused beam has a diameter √2 (about 1.414) times 2ω0. The beam-waist minimum is located at about the center of the crystal. It can be seen that, in this example LW is greater than the crystal length LC.
These relationships are dictated by the above-discussed goal of maximizing the overlap (volume ratio) in the crystal of the pump-beam and the seed-pulse beam for maximizing gain-extraction. In the example of FIG. 1, the overlap is about 70%. The relative large minimum beam-diameter is selected as discussed above to minimize thermal effects in the crystal, for minimizing seed-pulse beam aberration.
It is believed that a solid-state amplifier adaptable to a wide range of applications should be capable of handling pulse-energies up to 100 microjoules (μJ). In order to be effective for relatively low-power seed-pulses, for example, pulses having a pulse-energy of a few nanojoules (nJ), would require a gain-factor greater than 100, preferably between about 10,000 and 100,000. It is believed the even a gain-factor of 100 is far greater than has been hitherto achieved in a diode-pumped Nd:YVO4 solid-state amplifier. Applicants were not able to come close to producing such a high gain-factor by following the above-discussed “conventional wisdom” for solid-state amplifier design, at 878.6 nm or any other pump-radiation wavelength. Furthermore, even at modest levels of amplification, a degradation in the amplified beam quality was often observed, limiting the scalability of this prior-art approach.